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The process of cooling materials to cryogenic temperatures is often expensive and messy. One successful method is laser cooling, where photons interact with the atoms in some way to dampen their motion. While laser cooling of gases has been standard procedure for many years, solids are another issue entirely. Success has only come with a few specially prepared materials.

Having a laser annihilate something isn't usually associated with chilling anything down. But a new experiment reduced the temperature of a semiconductor by about 40°C using a laser. Jun Zhang, Dehui Li, Renjie Chen, and Qihua Xiong exploited a particular type of electronic excitation: when the photons interacted with this excitation, they canceled it out, damping the thermal fluctuations in the material.

Optical cooling

The cooling of materials using light was first proposed in 1929 by P. Pringsheim, well before the advent of lasers, but technical difficulties prevented its implementation. The principles were successfully combined with magnetic traps in subsequent decades, leading to the 1997 Nobel Prize in physics. Today, optical cooling is widely used in a number of applications, including Bose-Einstein condensation and atomic clocks.

Laser cooling of gases transfers some of the kinetic energy of the atoms into photons they interact with. Successful laser cooling was achieved in glasses—solids without an ordered, coherent crystal structure—by embedding rare-earth atoms in the matrix. As with gases, the excitation of the rare-earth atoms produced the cooling. However, that method won't work for every solid.

For solids, the thermal motion of the atoms takes the form of phonons: vibrations moving through the material. Being quantum excitations, phonons behave like particles: they can collide and scatter. One way to optically cool solids, therefore, would be to "annihilate" the phonons with laser light.

The authors of the new study used cadmium sulphide (CdS), a material known as a group-II-VI semiconductor. Commonly used in digital electronics, semiconductors are insulators under normal conditions, but can be induced to conduct electricity when impurity atoms are added. Group-II-VI semiconductors host both strong phonons, and an additional type of particle-like excitation known as an exciton. Excitons are created through interactions between electrons and "holes" that the electrons left behind.

The researchers fabricated narrow strips of CdS, deposited on a substrate of silicon and silicon dioxide at room temperature. They used an optical-wavelength laser, tuned to the precise wavelength to interact with multiple modes of phonons in the semiconductor. This interaction acted resonantly, canceling the phonons out—which means the material cooled rapidly, exhibiting a nearly 40°C drop in temperature.

The phonons in this material depend on temperature, so if it was colder to begin with, the laser wavelength needed to be longer—corresponding to lower energy. The researchers tested this and, while the temperature drop was less (about 15°C), the cooling process was more efficient.

To make sure it was resonant interaction between phonons and photons, the researchers used different laser wavelengths, and found they heated the CdS instead. Since these excitation modes are present in all group-II-VI semiconductors, the cooling method could be applied to other materials as well. Whether other, more common semiconductor materials can be cooled in similar ways isn't clear, but this experiment is a big step in the direction of rapid refrigeration of solids.